Copper (Cu) is the primary material used in integrated circuits (ICs) to create electrically conductive interconnects, and the etching of copper in ICs using focused ion beam (FIB) techniques is important in the field of circuit editing (CE) for failure verification and debugging of the ICs. Circuit editing of ICs with a focused-ion-beam (FIB) system requires that copper planes and traces be milled (cut) uniformly and cleanly so as to electrically isolate circuit elements across the separation created.
Achieving clean uniform FIB etching of copper for CE is difficult. In spite of significant past efforts to improve the quality of FIB copper etching processes, problems persist and improvements are needed in two main areas. These two areas are reducing electrically conductive re-deposited copper in the FIB etching area, and improving the etching selectivity and removal of copper while protecting the adjacent and underlying dielectric from unwanted etching and removal. Both of these problems arise from the properties of copper.
In contrast to aluminum (Al) that may be removed quickly and cleanly in a FIB operation using an agent such as iodine or an iodine-containing etchant, copper does not create simple volatile compounds with iodine or any other elements as does aluminum (e.g., AlI3) that can survive under ion bombardment and are easily removed. Rather, copper etching in a FIB process is due substantially exclusively to ion beam sputtering. This can inevitably lead to the unwanted re-deposition of conductive copper which causes problems as discussed below.
In addition, copper has a crystalline structure comprising crystal grains which have different crystallographic orientations. Grains with different orientations exhibit significantly different FIB etching rates and, therefore, FIB etching of copper is very non-uniform and uneven. It results in a very rough surface on the etched copper, and may perforate the copper layer down to the underlying dielectric. This may lead to significant damage of the underlying dielectric, and may result in electrical short circuiting of IC circuits from re-deposited electrically conductive copper. Past efforts to improving selectivity have focused principally on protecting the dielectric rather than enhancing the removal of copper, because the required selectivity that is achievable for copper etching is low relative to other materials, e.g., aluminum, where volatile etching byproducts are created.
Further, the ion beam sputtering process causes copper re-deposition all about the area exposed by the ion beam. This creates significant problems since the milled copper material that is intended to be removed re-deposits in this area, making electrical isolation of a signal difficult or impossible. In addition to affording protection of the adjacent and underlying dielectric against unwanted damage or removal, copper etch-assisting chemical agents must also actively assist in either limiting re-deposition of conductive copper material or converting re-deposited conductive copper to a non-conductive state to prevent unwanted electrical short circuits and ensure disconnection of copper conductors intended to be disconnected. FIG. 1 shows two comparative examples of FIB operations to cut copper lines using straight sputtering by an ion beam without any etch assisting agent (top trace A), and ion beam sputtering in an atmosphere of NH4OH comprising a mixture of oxidizing vapors of ammonia (NH3) and water (H2O) (bottom trace B). In the figures, copper appears as the bright areas. The different shades of brightness (gray to white) in the copper lines represent areas of different grain orientations that were etched differently.
In the top example A of FIG. 1, the re-deposited conductive copper is clearly seen as the thin bright lines 10, 11 on opposite sides of the severed copper line 12, demonstrating that electrical disconnection was not complete, i.e. there was no voltage difference between disconnected ends of the copper line. On the other hand, as shown in the bottom example B in the figure, the copper line 14 was successfully cut and the two ends of the line were disconnected electrically. The re-deposited conductive copper was oxidized by the vapors of NH3 and H2O, rendering it nonconductive and a voltage potential difference was present between the disconnected ends of the copper line. This example shows the advantages of using chemical oxidizers, such as electro-negative chemical elements like oxygen and nitrogen, as FIB etching agents for copper.
However, not all oxidizing agents will work with copper. For example, halogens should not be used as etch-assisting agents. Halogens, with the exception of fluorine, spontaneously react with and corrode copper without any activation by an ion beam, and they seriously degrade the conductivity of the copper. Even if halogen agents such as chlorine, bromine and iodine are carefully controlled, they can remain in the FIB vacuum chamber for a long time and continue to corrode any exposed copper. Furthermore, all halogens (including fluorine) are very aggressive to both high-k and low-k dielectrics.
When etching copper, there should be reasonably small and controlled over-etching of the dielectric so that performing CE on one metallization layer does not break through to an adjacent or underlying layer and create electrical leakage. Moreover, if cutting a copper line on a plane is not the last operation in the CE process, steps should be taken to ensure that the dielectric floor is flat after copper removal. Otherwise, subsequent operations can be seriously affected. As noted previously, because of its crystalline structure, the sputtering rate of copper is highly dependent on its grain orientation, and the average etching rate can vary significantly, as by a factor of four or more for a given set of FIB operating parameters. This is illustrated in FIG. 2 that shows the results of straight sputtering of copper 20 without any etch assisting chemistry applied, and clearly demonstrates the very uneven sputtering of copper due to its crystallographic structure. The dimensions of the milled area in the figure are approximately 10 μm×10 μm. As shown, the underlying dielectric is heavily damaged in areas where the copper removal was the greatest, while in other areas significant amounts of copper remain to be removed. From this figure, the difficulties in controlling the etching of copper may be readily appreciated.
Since copper removal is due to ion beam sputtering (not volatilization), copper removal occurs relatively slowly. This means that it is necessary to expose the copper to the ion beam for a longer period of time, and because of the unevenness of copper removal the ion beam may inadvertently damage some spots of the underlying dielectric. Thus, any copper etch assisting agent must provide protection of the adjacent or underlying dielectric to prevent unwanted etching of the dielectric. Once an area of dielectric is exposed, the etch assisting agent should either halt or significantly slow down the dielectric sputtering.
For conventional dielectrics, such as silicon dioxide, SiO2, it has been found that oxygen, water or a mixture of vapors of water and ammonia can slow down dielectric sputtering by up to a factor of ten if the ion beam current density and vapor pressure are adjusted properly. Water and ammonia are good oxidizers and have been found to afford reasonably good protection for conventional dielectrics, and oxygen, water and a mixture of water and ammonia have been used as copper etch assisting chemicals for etching copper over conventional SiO2 dielectric. However, these compounds have been found to be useless for protecting the new low-k dielectrics being increasingly used in ICs. The main problem is that many low-k dielectrics contain carbon as one of the main components of the dielectric structure, which is why low-k dielectrics are sometimes called “organic” dielectrics. Both water and oxygen easily oxidize carbon in the dielectric structure to produce carbon monoxide (CO) or carbon dioxide (CO2), both of which are gases and are volatilized. Therefore, rather than being protective agents for the dielectric, they accelerate dielectric etching by volatilizing one of the main components of the dielectric structure.
There has been and is an increasing tendency in the IC industry to employ dielectrics with even lower k numbers by increasing of the proportion of carbon in their structures. This has led to a demand for new copper etch assisting chemistries that are capable of protecting the dielectric. U.S. Pat. No. 7,060,196 discloses and claims a number of chemicals, mainly nitro-compounds such as Nitro-methane, Nitro-ethane, Nitro-propane, Nitro-ethanol and others, for use as etch assisting agents to protect dielectrics in FIB copper etching applications. Nitro-ethanol presently is one of the most widely used chemical agents for etching copper over organic dielectrics in FIB operations. While Nitro-ethanol has been effective in limiting dielectric etching, it has not been very effective addressing the problem of re-deposition of sputtered conductive copper material on surfaces adjacent to the IC work area. This is shown in FIG. 3.
FIG. 3 illustrates the results of etching copper over Black Diamond™ dielectric using Nitro-ethanol as an etch assisting agent. As shown, the center of the milled area has a flat bottom with two rows of contacts 30 comprising vias connected to the next lower layer. However, re-deposited copper is clearly present on the vertical walls of the openings, as indicated at 32. Re-deposited conductive copper material appears as bright areas in the figure as it produces secondary electron emission. The re-deposited material is conductive because it contains copper and some carbon from the copper etch assisting compound (Nitro-ethanol in this case). This re-deposited material may render the IC partially or totally inoperative by electrically short circuiting interconnects or grounding copper power planes. For example, if in the figure the upper milled area 34 were not wider than the lower milled area 36, the top and bottom power planes would be electrically shorted to each other by the re-deposited conductive copper on the vertical walls of the openings. There is also more re-deposited conductive copper material in the milled areas which is invisible in the figure because it is not grounded and does not produce secondary electron emission. From the figure, it can be concluded that while the Nitro-ethanol copper etch assisting compound functions well to protect the underlying dielectric, it is not effective in addressing conductive copper re-deposition.
Thus, there is a need for etching methods and etch assisting agents for FIB etching of copper that addresses the foregoing and other problems with known methods and etch assisting agents. In particular, there is a need for etching methods and etch assisting agents for FIB etching of copper that both protect the adjacent and underlying dielectric from unwanted etching, and that avoid the problems caused by re-deposition of conductive copper and other materials. It is to these ends that the present invention is directed.